Optical compensation using a space division multiplexing electro-optic receiver

ABSTRACT

Disclosed herein are methods, structures, and devices for optical communications systems operating through turbulent media. More specifically, a spatial division multiplexing photonic integrated circuit is used in conjunction with digital signal processing systems to mitigate the effects of the turbulent media.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 61/722,223 filed Nov. 4, 2012 which is incorporatedby reference in its entirety as if set forth at length herein.

TECHNICAL FIELD

This disclosure relates generally to the field of optical communicationsand in particular to optical communications systems operating throughturbulent media.

BACKGROUND

Optical communications systems (e.g. systems that use lasers or otherfree space techniques) oftentimes must operate through turbulent media.Accordingly, methods, apparatus and structures that improve theoperation of such systems in turbulent media would represent a welcomeaddition to the art.

SUMMARY

An advance in the art is made according to an aspect of the presentdisclosure directed to methods, structures that mitigate the effects ofturbulent media on the effectiveness and efficiency of opticalcommunications systems. More specifically, this disclosure describesphotonic circuits and techniques that perform space divisionmultiplexing in combination with adaptive digital signal processingtechniques to provide such mitigation.

Viewed from a first aspect, the present disclosure is directed tomethods and structures that collect light through the effect of receiveroptics and direct that received light to a multimode optical amplifierwhich outputs amplified light that is further directed to photonicintegrated circuit(s) where it is received and undergoes spatialdivision multiplexing and photodetection. Outputs of the photodetectorsare sent to receiver electronics including transimpedance amplifiers(TIA), automatic gain control (AGC), analog to digital conversion (ADC)and/or other electronics. Various signals undergo digital signalprocessing such that adjustments are made to amplitude(s), phase(s), anddelay(s) of the signals and subsequently combining them to enhance areceived signal from which data is determined.

Viewed from another aspect, the present disclosure is directed tomethods and structures that collect light through the effect of receiveroptics, combine the received light with local oscillator laser light,then applying the combined light to photonic integrated circuit(s) thatperform spatial division multiplexing and photodetection.

BRIEF DESCRIPTION OF THE DRAWING

A more complete understanding of the present disclosure may be realizedby reference to the accompanying drawings in which:

FIG. 1 shows a schematic illustration of an exemplary prior art opticalcommunications system operating through turbulent media;

FIG. 2 shows a schematic illustration of a exemplary prior art adaptiveoptical system that employs a deformable mirror used to mitigateturbulence effects in an optical communications system;

FIG. 3 shows a schematic illustration of an exemplary prior art opticalcommunications system receiver that employs a multimode opticalamplifier and local oscillator laser in combination with a multimodephotodetector;

FIG. 4 shows a schematic illustration of an exemplary opticalcommunications system according to the present disclosure that employs aphotonic integrated circuit that performs spatial division multiplexing,electrical processing and digital signal processing wherein (a) amultimode optical amplifier is used and (b) a local oscillator is used;

FIG. 5 shows a schematic illustration of exemplary low order fiber modesin a low order multimode fiber;

FIG. 6 shows a schematic illustration of an exemplary photonicintegrated circuit according to the present disclosure that employssurface grating couplers and other techniques to collect multipleoptical fiber modes such as those shown in FIG. 5 and spatially separatethem into separate substantially orthogonal optical channels and performphotodetection on each channel separately;

FIG. 7 shows a schematic illustration of an exemplary photonicintegrated circuit according to the present disclosure that employssurface grating couplers coupled to waveguides to collect orthogonalspatial data;

FIG. 8 shows a schematic illustration of an exemplary adaptive digitalsignal processor according to an aspect of the present disclosure; and

FIG. 9 show a schematic illustration of an exemplary opticalcommunications system including a receiver telescope array for receivingoptical signals after traversing turbulent media according to yetanother aspect of the present disclosure.

DETAILED DESCRIPTION

The following merely illustrates the principles of the disclosure. Itwill thus be appreciated that those skilled in the art will be able todevise various arrangements which, although not explicitly described orshown herein, embody the principles of the disclosure and are includedwithin its spirit and scope. More particularly, while numerous specificdetails are set forth, it is understood that embodiments of thedisclosure may be practiced without these specific details and in otherinstances, well-known circuits, structures and techniques have not beshown in order not to obscure the understanding of this disclosure.

Furthermore, all examples and conditional language recited herein areprincipally intended expressly to be only for pedagogical purposes toaid the reader in understanding the principles of the disclosure and theconcepts contributed by the inventor(s) to furthering the art, and areto be construed as being without limitation to such specifically recitedexamples and conditions.

Moreover, all statements herein reciting principles, aspects, andembodiments of the disclosure, as well as specific examples thereof, areintended to encompass both structural and functional equivalentsthereof. Additionally, it is intended that such equivalents include bothcurrently-known equivalents as well as equivalents developed in thefuture, i.e., any elements developed that perform the same function,regardless of structure.

Thus, for example, it will be appreciated by those skilled in the artthat the diagrams herein represent conceptual views of illustrativestructures embodying the principles of the invention.

In addition, it will be appreciated by those skilled in art that anyflow charts, flow diagrams, state transition diagrams, pseudocode, andthe like represent various processes which may be substantiallyrepresented in computer readable medium and so executed by a computer orprocessor, whether or not such computer or processor is explicitlyshown.

In the claims hereof any element expressed as a means for performing aspecified function is intended to encompass any way of performing thatfunction including, for example, a) a combination of circuit elementswhich performs that function or b) software in any form, including,therefore, firmware, microcode or the like, combined with appropriatecircuitry for executing that software to perform the function. Theinvention as defined by such claims resides in the fact that thefunctionalities provided by the various recited means are combined andbrought together in the manner which the claims call for. Applicant thusregards any means which can provide those functionalities as equivalentas those shown herein. Finally, and unless otherwise explicitlyspecified herein, the drawings are not drawn to scale.

Thus, for example, it will be appreciated by those skilled in the artthat the diagrams herein represent conceptual views of illustrativestructures embodying the principles of the disclosure.

By way of some additional background, and with initial reference to FIG.1 which depicts in schematic form an exemplary prior art opticalcommunications system operating through turbulent media. As shown inthat figure, data is transmitted by a transmitter system using atransmitter telescope through turbulent media. A receiver systemreceives the transmitted data via a receiver telescope and recovers thedata for subsequent use. As will be readily appreciated, such opticalcommunications systems (for example, those that use lasers) oftentimesmust transmit through turbulent media—such as the atmosphere or othermedia—which may produce wavefront or other distortions.

In such environments, complex electro-optical-mechanical systems such asthe deformable mirror adaptive optical system shown schematically inFIG. 2 may be employed. Shown in that figure are plane waves distortedby a wavefront-distorting medium (such as the turbulence depicted inFIG. 1). The distorted wavefront(s) are corrected through the effect ofa deformable mirror that is adjusted by wavefront sensor and dataprocessor/driver system such that a corrected, compensated image may beformed. As may be readily appreciated, such complex systems may be quitecostly and may further require beacon lasers or other techniques toeffectively compensate for the undesirable distortions describedpreviously.

As may be appreciated by those skilled in the art, when very high speedoptical communications systems require high sensitivity it is beneficialto employ optical amplifiers or coherent detection as depictedschematically in FIG. 3 which shows a system employing a multimodeoptical amplifier (FIG. 3( a)) and a system employing a coherent localoscillator laser (FIG. 3( b)).

Notably, there is a difficult tradeoff to be made when the telescopediameter is increased to collect more power and experience an improvedsignal to noise ratio as it is accompanied by the degradation thatoccurs from having a multimode receiver. More specifically, if the modesare not combined intelligently, then performance can be significantlyworse than the situation where there is no atmospheric turbulence and asingle mode receiver used.

As depicted and shown previously in FIG. 1, optical communicationssystems that transmit through turbulent channels—such as through theatmosphere, or fluids such as water—are challenged to collect light froma distant transmitter wherein the phase front is distorted and varyingin time due to changing channel conditions. Oftentimes, such turbulentchannels are characterized in terms of the nominal transverse coherencelength (R₀). If a receiver telescope is significantly smaller than R₀,then dominant effect(s) of the turbulence includes tilt and fading.Fortunately—and as readily known in the art—tilt may be corrected usingangular beam steering devices and spatial tracking systems.

Those skilled in the art will appreciate that with systems such as thosedepicted in FIG. 1, it is oftentimes desirable to increase the telescopeaperture to collect more light such that better estimates of receiveddata may be made. Such increase in aperture is desirable—forexample—when long links such as links to/from space to/from Earth and/orvery high speed links.

Of particular interest, as the telescope diameter of such systemsincreases, the wavefront across the telescope aperture is no longersimply composed of tilt. This characteristic poses additional problemsfor receivers that employ optical amplifiers or coherent localoscillators or their accompanying methods. More particularly, suchreceivers exhibit improved sensitivity in a single spatial mode but notwhen more than one spatial mode is collected at a time. If only a fewspatial modes are detected, it may be possible to employ such techniquesas those depicted in FIG. 3, however as the number of spatial modesincreases or one needs to minimize deep signal fades, additionaltechniques are necessary.

With these principles in place, we now turn to FIG. 4 which illustratesschematically structures according to the present disclosure in which aspatial division multiplexing circuit(s) is used in combination with amultimode optical receiver (4(a)), or with a local oscillator heterodyneor homodyne technique (4(b)). For example, if a multimode opticalamplifier is employed the first few fiber modes may be represented suchas that shown in FIG. 5.

Advantageously, and according to an aspect of the present disclosure, aphotonic integrated circuit employing advanced optical components suchas surface grating couplers, beam splitters, beam combiners, opticalamplifiers, phase shifters and other optical elements such as thoseshown in FIG. 6 and FIG. 7, may be employed to spatially separate andcollectively process optically and/or electrically the substantiallyorthogonal spatial modes. Once such processing is performed, additionaldigital signal processing techniques may then be employed to adjust theamplitude, phase and delay, and then to combine the various modes intoone output (or two in the case of polarization division multiplexedtransmitters) such that the combination results in mitigated effects ofany turbulent channels. In particular situations, such processing mayresult in performance characteristics of an optical channel undergoingno turbulence when used with a signal spatial mode receiver.

As may be readily understood, there are a number of ways to extractnearly orthogonal modes from a multimode receiver. One such way tocreate and/or receive phase-patterned beam(s) is to employ gratingcouplers. As those skilled in the art will understand, a grating coupleris a periodic pattern in a vertically high-index-contrast waveguide—suchas silicon-on-insulator (SOI) waveguide—in which light scattered fromthe grating may be phase matched to emit substantially vertically fromthe surface of a photonic integrated circuit. Preferably, the period ofthe grating is approximately to the effective wavelength of thewaveguide.

In now describing further exemplary embodiments of systems and methodsaccording to aspects of the present disclosure, we assume that suchsystems are reciprocal and that when we describe a beam emanating fromthe grating coupler, it is understood that the system may operate in areciprocal or reverse direction. Accordingly, such systems may receive abeam traveling in an opposite direction to a beam transmitted.

Turning now to FIG. 6, there is shown an exemplary grating coupleraccording to an aspect of the present disclosure. More specifically, thegrating coupler shown is a circular grating coupler that includesgrooves arranged in substantially concentric circles. Shown further isan array of radially-arranged waveguides that may be used to directlight to the circular grating. These waveguides may be further connectedto one or more input/output waveguides by one or more optical couplers.

As may be appreciated a number of controllable phase shifters may beoptionally positioned within the radial waveguides such that theazimuthal phase distribution emanating from the grating (coupler) iscontrollable. However, one cannot control a radial phase distributionvia such control of the waveguide phases. Consequently, and according toan aspect of the present disclosure, phase shifters may beadvantageously positioned in the grating coupler in a circular pattern.

By way of example, and with continued reference to FIG. 6, there isshown a number of tunable phase shifters positioned inside the gratingin a circular pattern. More specifically as one examines the gratingdepicted in that figure, it may be observed that the overall structureincludes a number of concentric grating grooves with a number of phaseshifters interposed therein concentrically at predetermined positions.Advantageously, such a structure provides the extraction of orthogonalangular momemtium modes and is efficient for turbulence that exhibitssubstantial circular symmetry.

In an exemplary embodiment and as shown as an inset in FIG. 6, thecircular grating coupler array radial waveguides may be opticallyconnected to an array of waveguides which comprise an array of outputwaveguides from a structure such as a star coupler. The input(s) of thatstar coupler may be coupled to an array of inputs. Shown further in thatinset are a number of operational phase shifters for azimuthal controldescribed above positioned within connecting waveguides.

Yet another approach to turbulence compensation is depicted in FIG. 7,which shows an example of a photonic integrated circuit having surfacegrating couplers and orthogonal spatial modes. As may be observed, a2-Dimensional pattern of individual grating couplers may be curved withthe curvature substantially centered on an inlet of a “feeding”waveguide. Notably, such “feeding” waveguides provide light to theparticular curved grating that it feeds. Of further advantage the radiusof curvature and length of the curve may be varied as dictated byparticular requirements.

As shown in FIG. 7, the grating couplers are arranged in a triangularlattice, which is a 2-D lattice exhibiting the highest packing density.The waveguides feeding the individual grating couplers are formedbetween adjacent, individual gratings which may limit the size of theoverall grating. Notwithstanding, if one needs a larger array orlattice, then arrangements wherein feeding waveguides from subsectionsare formed in a lower waveguide layer. Furthermore, a wide variety ofgratings may be designed that operate with one waveguide or multiplewaveguides per grating element. Such waveguides may be used to extractsignal polarization or include multiple polarizations that maysubsequently be separated using waveguide polarization splitters.

With reference now to FIG. 8, there it shows a schematic illustration ofexemplary data processing to recover a transmitted optical signalaccording to an aspect of the present disclosure. And while forsimplicity we describe a case involving a single transmittedpolarization state, the principles according to the present disclosuremay be equally applied to those case(s) wherein two orthogonal states ofpolarization are transmitted, each carrying independent data.

As shown schematically in FIG. 8, as a received, distorted wavefront isreceived through the effect of receiver optics, it is imaged onto andcaptured by an array of receptors. At this point we note that thestructures described are preferably formed onto/into a single photonicintegrated circuit (PIC), although different requirements may dictatedifferent arrangements. We note further that the receptor array depictedin the figure shows six (N=6) receptor elements, but those skilled inthe art will understand that that number may be changed as well asrequirements dictate.

Once the received optical wavefront has been imaged onto and captured bythe array of receptors on the PIC, it may be processed for enhanced(e.g., optimal) signal reception. As depicted in the figure, the seriesof graphics at point “A” in the figure illustrate that the individualsignal components x_(i) produced by the receptors may vary in phase,amplitude and possibly delay since the received phase front is notuniform. As may be appreciated, one goal of signal processing is tomanipulate these individual signals to y_(i) and bring them into“alignment”—as indicated by the series of graphics at point “B” in thefigure. When so aligned, the composite received is maximized withrespect to any noise. Equalizers, positioned in each path mayadvantageously perform correction to each of the individual signals.

The signals are combined and—in the case of coherent detection such asthat shown here—phase recovery is performed to compensate for the phasedifference(s) between transmit and receive optical carriers. Finally,pre-determined decision criteria are used to determine/extract any datafrom the signal.

Advantageously, the signal equalization may be performed in either theoptical domain before any photodetection, or in the electrical domainafter photodetection. In the case of the former, optical phases may beadjusted with optical elements such as tunable couplers and phaseshifters and delays. FIG. 8 however, illustrates the latter case wherethe individual optical signals are down-converted to an electricalbaseband by mixing them with a common optical local oscillator anddetecting with a combination of 90-degree optical hybrids and a pair ofbalanced photodetectors to produce a complex baseband signal.

After digitization by analog-to-digital converters (ADCs), theequalization may be performed numerically via digital signal processors(DSP). The equalizers illustrated in the FIG. 8 may be considered assingle complex multipliers w₁. Advantageously, this allows the phase andamplitude of the signals to be adjusted. In certain situations there maybe delays between pulses (arising for example, from dispersion in anymultimode fiber). One solution for such situations is to use equalizershaving multiple taps, each delayed in time to form a finite impulseresponse (FIR) filter that can also time shift and reshape the pulse.

As may be appreciated, particular methods and/or algorithms are usefulto automatically adjust the equalizer w_(i) to enhance or optimize thereception of the signal. Furthermore, such algorithms may continuallyrun to adapt to changes in the transmission medium and any changes inturbulence.

Fortunately, there exist a number of techniques for such adaptiveequalization. Such methods may be run either “blind”—that is, using onlythe received data information—or, be guided by inserted trainingsequences.

Methods employing training sequences may advantageously offer enhancedrobustness however accompanied by additional complexity. Generally, withblind optimization, the equalized signal y is compared against areference to determine an error, or cost metric, e. The value of e canbe viewed as a surface in multi-dimensional space as function of w_(i).The goal is to adjust equalizer weights w_(i) in order to effectivelyminimize e.

If the direction of gradient at the current location w_(i) is know, thenw_(i) are updated to new values so as to head “downhill” towards theminimum. In practice however, an analytical expression for the gradientof the error may not be available, and various methods have beendeveloped to achieve this.

One common method of estimating the gradient and equalizer update is theLeaset Mean Square (LMS) method illustrated in FIG. 8. As a reference,LMS may use the detected signals (decision directed mode) or knowntransmitted “training signals”. Choice of algorithm may depend on themodulation format for the data. The Constant Modulus Algorithm (CMA) isa common algorithm that exploits the properties of phase-modulatedsignals since the desired signal is known to lie on a constant radius.Such CMA algorithms are successful in blind convergence. Generally, theequalizer update step for all methods involves a step-size parameter μ(mu) that controls how aggressively the algorithm descends to theminimum. The more aggressive the convergence, the faster the tracking isbut more prone to instability and noise. Implementing an appropriatealgorithm enables the receiver to optimize reception and adaptivelytrack out variations in the optical phase front caused by transmissionthrough turbulent media.

Finally, and with reference to FIG. 9, there it shows another approachto compensation according to an aspect of the present disclosure. Asshown in that figure, a telescope array is used to collect the opticalsignals which traverse the turbulent media. The signals so collected arethen applied to a multi-core optical amplifier. The amplifier is in turncoupled to a silicon PIC array that may employ surface grating couplersthat may advantageously be butt coupled to the array fiber amplifier.The PIC is then—in turn—coupled to a high speed ASIC processor forfurther processing and data extraction.

At this point, those skilled in the art will readily appreciate thatwhile the methods, techniques and structures according to the presentdisclosure have been described with respect to particularimplementations and/or embodiments, those skilled in the art willrecognize that the disclosure is not so limited. Accordingly, the scopeof the disclosure should only be limited by the claims appended hereto.

1. A method of compensating an optical signal that has been transmittedthrough a turbulent medium comprising the steps of: receiving throughthe effect of receiver optics, the optical signal that has beentransmitted through the turbulent medium; compensating for theturbulence by spatial-division-multiplexing the received optical signalusing a photonic integrated circuit; and processing thespatial-division-multiplexed signal such that any effects of theturbulence is mitigated; recovering any data conveyed by the opticalsignal.
 2. The method of claim 1 wherein thespatial-division-multiplexing produces a plurality of output signals andthe processing is performed on that plurality of output signals.
 3. Themethod of claim 1 further comprising the step of amplifying the receivedoptical signal that has been transmitted through the turbulent mediumthrough the effect of a multimode optical amplifier.
 4. The method ofclaim 1 further comprising the step of combining the received opticalsignal with a signal generated by a local oscillator laser.
 5. Themethod of claim 1 wherein said spatial division multiplexing isperformed by the photonic integrated circuit through the effect of acircular grating coupler.
 6. The method of claim 5 wherein said circulargrating coupler includes a plurality of concentric grooves and aplurality of radially oriented waveguides disposed around the perimeterof the concentric grooves.
 7. The method of claim 6 wherein each one ofsaid radially oriented waveguides are positioned in a respective opticalpath including one or more phase shifters for azimuthal control.
 8. Themethod of claim 6 wherein said circular grating coupler includes one ormore phase shifters positioned concentrically within and interposedbetween particular ones of the concentric grooves.
 9. The method ofclaim 1 wherein said turbulence is caused by atmospheric conditions. 10.The method of claim 1 wherein said spatial division multiplexing isperformed by the photonic integrated circuit through the effect of anarray of gratings arranged in a triangular lattice.
 11. The method ofclaim 1 further comprising the step of amplifying the received opticalsignal that has been transmitted through the turbulent medium throughthe effect of an array of multimode optical amplifiers and wherein saidreceiver optics includes a corresponding array of telescopes.
 12. Anoptical communications system for receiving an optical signal that hasbeen transmitted through a turbulent medium, said system comprising:receiver optics, for receiving the optical signal that has beentransmitted through the turbulent medium; a photonic integrated circuitthat performs spatial-division-multiplexing on the received opticalsignal; and a processing element that processes thespatial-division-multiplexed signal such that any effects of theturbulence is mitigated; a data extractor that extracts any data fromthe processed signal.
 13. The system of claim 12 wherein the photonicintegrated circuit produces a plurality of output signals and theprocessing elements operates on that plurality of output signals. 14.The system of claim 13 further comprising a multimode optical amplifierthat amplifies the received optical signal that has been transmittedthrough the turbulent medium.
 15. The system of claim 12 furthercomprising a local oscillator laser which produces an output light thatis combined with the received optical signal prior to thespecial-division-multiplexing.
 16. The system of claim 12 wherein thephotonic integrated circuit includes a circular grating coupler foreffecting the spatial-division multiplexing.
 17. The system of claim 16wherein said circular grating coupler includes a plurality of concentricgrooves and a plurality of radially oriented waveguides disposed aroundthe perimeter of the concentric grooves.
 18. The system of claim 17wherein each one of said radially oriented waveguides are positioned ina respective optical path including one or more phase shifters forazimuthal control.
 19. The system of claim 18 wherein said circulargrating coupler includes one or more phase shifters positionedconcentrically within and interposed between particular ones of theconcentric grooves.
 20. The system of claim 12 wherein said turbulenceis caused by atmospheric conditions and said receiver optics includes atelescope array, an array of multicore fiber amplifiers and saidphotonic integrated circuit includes a grating array arranged in atriangular lattice.